This invention relates to casting of metal sheet or strip, and more particularly to an improved method of and apparatus for the high speed direct casting of thin metal sheet or strip in a continuous or semi-continuous operation.
The metals industry has, for many years, sought to develop a commercially acceptable process for the direct casting of metals into thin sheet or strip (hereinafter, strip) in a semi-continuous or continuous operation. These efforts have been intensified in recent years and substantial research and development has been sponsored both by industry and by various governments. For example, the U.S. Department of Energy has awarded a 2.6 million dollar contract to Westinghouse Electric Corporation and Armco Inc. jointly for development of a roller casting process for forming strip only 3 inches in width, and a 30 million dollar contract to United States Steel Corporation and Bethlehem Steel Corporation jointly for the design and construction of a pilot plant for strip casting.
Efforts to develop a commercially acceptable process for direct casting metal strip have continued throughout most of this century. For example, British patent No. 6,630 discloses the concept of flowing molten metal at a constant rate onto a moving cooled surface to be solidified, and drawing the metal in the form of a thin strip from the cooled surface in a continuous process. Other patents disclosing and describing improvements and variations of this basic concept include, for example, U.S. Pat. Nos. 4,479,528; 4,449,568 and 3,381,736.
A variation of the above concept involves conducting molten metal from a tundish through a restricted outlet so as to provide a convex meniscus at the outlet opening, with the molten metal being drawn from the opening by contacting the meniscus with a moving cooled surface. Examples of patents disclosing this concept include British patent No. 20,518 and U.S. Pat. Nos. 3,522,836 and 3,605,863.
The concept of flowing a stream of molten metal into the nip of a pair of spaced, counter-rotating chilled rolls to produce an elongated strip rolled and chilled on both surfaces is disclosed, for example, in U.S. Pat. Nos. 4,212,344 and 4,337,087 and is described as known prior art in Japanese Published application No. 58-41656.
The use of a chilled roll surface partially submerged in a molten metal bath and driven to withdraw a strip or filament of metal solidified and adhered to the chilled surface is disclosed in U.S. Pat. Nos. 3,540,517 and 3,812,901. The use of a pair of counter-rotating rolls having chilled surfaces partially submerged in a molten metal bath to withdraw a continuous strip through the nip of the rolls is shown in U.S. Pat. Nos. 3,823,762 and 3,857,434.
The use of travelling molds in the form of endless chilled belts or of chilled mold sections connected in caterpillar-track fashion for the continuous casting of metal is also known and is commercially used in the production of plate or thin slabs. Such devices may use a single casting mold or belt as shown in U.S. Pat. Nos. 2,348,178; 3,381,739; 4,274,473; and 4,323,419, or a pair of cooperating endless molds or belts, as shown in U.S. Pat. Nos. 3,642,055 and 4,061,177. The combination of an endless belt or casting mold with a roll contacting the opposed surface of the cast strand is also known as shown, for example, in U.S. Pat. Nos. 4,202,404 and 4,372,368, Swiss patent No. 622,725, and French patent No. 1,364,717.
The use of a gas jet directed onto the free surface of a pool of molten metal to create a standing wave contacting a cooled casting wheel surface is disclosed in U.S. Pat. No. 3,863,700.
It is also known to spray a cooling fluid such as an inert gas or a cooling liquid onto the exposed molten surface of a partially solidified band of metal moving on a chilled casting surface to enable the continuous casting of a thicker gauge plate and to enable subsequent shaping of the exposed top surface by a shaping roll, as illustrated for example in the published Japanese application and the Swiss patent mentioned hereinabove.
U.S. Pat. No. 4,282,921 discloses a process for melt spinning narrow metallic ribbons by directing a jet of molten metal onto a moving chill block surface and directing a gas stream confluent with and surrounding the molten metal jet so that the gas surrounds and bears upon the metal puddle near the point of impingement on the moving chilled surface to stabilize the puddle as the ribbon is formed.
The use of air knives for controlling the thickness of a liquid metal coating on a solid substrate is well known and widely used particularly in hot dip galvanizing and aluminizing of metal strip. Such air knives conventionally include an elongated hollow manifold extending transversely of and in close proximity to the emerging elongated substrate at a point spaced above the coating metal bath. An elongated, narrow nozzle opening extends along the manifold and faces in the direction of the coated substrate. Gas, under pressure, discharged from the nozzle acts as a pressure dam which, depending upon characteristics of the jet including the direction, velocity and mass of the gas and the proximity of the nozzle outlet to the liquid coating material, limits the thickness of the liquid coating carried past the air knife.
In such hot dip metal coating operation, the metal substrate is conventionally passed through a preheating furnace and led directly into the molten metal bath so that the substrate is at a temperature which will maintain the coating in a liquid state for a substantial distance past the air knife, with solidification normally taking place from the exposed coating surface inward. A detailed technical description and analysis of the hot dip metal coating on metal strip is presented by John A. Thornton and Hart F. Graff, An Analytical Description of the Jet Finishing Process for Hot Dip Metallic Coatings on Strip, Metallurgical Transactions B, pages 607-618, December 1976.
Despite the continued efforts by the metals industry, applicant is unaware of any previous process which is capable of producing commercially acceptable cast metal strip in a continuous high speed process. It is to be understood that the term "continuous" as used herein is intended to include a semi-continuous direct strip casting operation.
It is the primary object of the present invention to provide an improved process for the direct continuous casting of metal strip.
It is another object of the present invention to provide an improved apparatus and process for continuously producing thin cast metal strip utilizing a melt drag technique.
Another object is to provide apparatus for and method of continuously casting thin metallic strip having a more uniform cross sectional shape and a good top surface finish.
Another object is to provide such a method and apparatus which is capable of high speed production of cast metal strip in a continuous commercial operation.
Another object is to provide a method and apparatus for the high speed casting of metal strip having a thickness which can be varied over a wide range.
Another object is to provide such a method and apparatus for producing cast metal strip which is substantially free of surface defects and inclusions.
The foregoing and other objects and features of the invention are achieved in accordance with the present invention which enables metal strip to be cast from a supply of molten metal by a continuous process wherein the molten metal is brought into contact with a cooled moving casting surface whereby a continuous strand of the metal solidifies on and adheres to the cooled surface to be withdrawn from the molten metal supply. An air knife supported adjacent to the surface of the molten metal supply directs a thin low pressure air jet into contact with the surfaces of the molten metal supply and the emerging strand at the point of emergence of the solidifying strand. The position and direction of the fluid jet, the shape of the outlet nozzle and the gas pressure are controlled to shape the free surface of the strand as it exits from the molten metal supply and to prevent oxides, slag or other material on the molten metal supply surface from adhering to liquid metal on the surface of the strand while at the same time controlling strip thickness and strip profile by limiting the amount and distribution of liquid metal adhering to the free top surface of the partially solidified strand.
In accordance with a preferred embodiment of the invention, the casting surface is the cylindrical outer surface of a casting wheel or drum which is supported and driven for rotation about a fixed horizontal axis. A tundish supported adjacent to the casting wheel has an open end contoured to fit in close conformity with and be effectively closed by a portion of the casting wheel surface. Molten metal is continuously supplied to the tundish to maintain a substantially constant depth of molten metal in contact with the rotating chilled casting surface. As the casting surface moves upward through the molten metal the metal wets and adheres to the chilled surface and is quickly solidified, with the solidified strand increasing in thickness progressively until it emerges from the top surface of the metal in the tundish.
A gas discharge nozzle assembly, hereinafter referred to an an air knife, is supported above the surface of the metal in the tundish and has an elongated nozzle outlet positioned and oriented to direct a low speed jet of gas onto the molten metal surface along the line of intersection of the surface of the molten metal and the casting surface, i.e., the point at which the strand emerges from the molten metal. The gas jet establishes a depression in the surface of the molten metal in the tundish adjacent to the emerging strand and produces a standing wave adjacent to the depression. This standing wave and the gas from the jet sweeping over its surface prevent oxides, slag and other material on the molten metal surface from contacting the exposed wet surface of the emerging strand adhering to and moving with the cooled casting surface.
The fluid jet has a component of motion generally perpendicular to the exposed surface of the strand at the point of emergence from the molten metal in the tundish. At this point, the strand is substantially solidified, but the exposed top surface has a layer of liquid metal adhering thereto. The velocity, mass and direction of the gas in the jet are controlled so that the gas acts upon and shapes this liquid layer to produce the desired cross-sectional shape and surface finish. The jet is also effective to limit the thickness of the liquid layer and to this extent the thickness of the strand. As previously stated, however, the emerging strand is solidified throughout at least a major portion of its thickness and the final strip thickness will be determined by a combination of factors.
The chilled casting wheel surface rapidly quenches the metal contacting the surface so that the strand is completely solidified very quickly after passing beyond the jet from the air knife. Thus, the top surface finish and shape imparted to the strip by the controlled jet is maintained as a result of the rapid solidification. Strip thickness can also be varied independently of the air knife by varying the time of exposure of the molten metal to the chilled casting wheel surface by either varying the speed of the casting wheel or the length of the wheel surface which at any given time is exposed to the molten metal in the tundish.